Question: Which of these numbers is prime? ${3,\ 45,\ 49,\ 51,\ 84}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 3 are 1 and 3. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 49 are 1, 7, and 49. The factors of 51 are 1, 3, 17, and 51. The factors of 84 are 1, 2, 3, 4, 6, 7, 12, 14, 21, 28, 42, and 84. Thus, 3 is a prime number.